Refactor helper for file input across uutils

Across many of the bins there's the pattern that follows:

        let buffer = BufReader::new(if filename == "-" {
            stdin_buf = stdin();
            &mut stdin_buf as &mut dyn Read
        } else {
            file_buf = File::open(Path::new(filename)).map_err_context(|| filename.to_string())?;
            &mut file_buf as &mut dyn Read
        });

Was hoping to make a helper in uucore and standardize this process of reading from either the stdin or the file path and use it everywhere across the project. Some of them have the map_err_context and some don't the goal would be to add it to all of them.

Start by inventorying the repeated stdin-or-file reading pattern across the bins and inspect the uucore entry points where a shared helper could live. Standardize those call sites so file-opening errors consistently include context, then verify that all identified bins use the common process.